1. Field of the Invention
The present invention relates to bicycle suspension systems and more particularly to a suspension fork assembly. This invention is an improvement on the system first described by Farris et al. U.S. Pat. No. 5,320,374 and subsequent patents (U.S. Pat. Nos. 5,494302, 5,702,092, 5,924,714, 6,007,056, and 6,155,541). The related patents address the geometric configuration of two tubes, one inside the other, both containing several flats permitting the insertion of hardened races and roller needle bearings between the two tubes. The resulting telescoping action provides longitudinal motion, but prohibits torsional movement. A key objective is to achieve a smooth, plush feel of the telescoping action.
2. Description of the Prior Art
Various suspension systems have been proposed and developed for bicycles. Many of these systems utilize a pair of telescoping assemblies between which the front wheel is mounted. Each assembly comprises an outer tube and an inner tube which is free to move in and out of the outer tube and is cushioned by a damper of one sort or another. The outer tubes are connected at the lower ends to the bicycle axle of the front wheel and the upper ends of the inner tube are connected together in a fashion similar to the usual upper end of a bicycle fork.
As is known to those skilled in the art, these types of suspension systems use anti-friction bushings to allow free movement of the inner tube within the outer tube. These bushings have undesirable static friction called xe2x80x9cstiction.xe2x80x9d Because of this, the suspension systems using such bushings tend to stick and release. In addition, the two telescoping assemblies also have to be fixed together in some manner as through a xe2x80x9cUxe2x80x9d shaped yoke at the upper ends of the tubes to eliminate twisting. Even with this xe2x80x9cUxe2x80x9d shaped yoke the torsional stiffness of these types of assemblies is still limited.
A prior art example of a system which overcomes this issue is shown in Farris et al. U.S. Pat No. 5,320,374 and subsequent applications. In this example an improved form of suspension system is described using an outer tube which is adapted to be mounted in and extend through the head tube of the bicycle frame and an inner tube connected to the fork of the bicycle which telescopes within the outer tube. The inner surface of the outer tube and the outer surface of the inner tube each have a plurality of axially arranged opposing longitudinal flat sections such as four on each tube. A plurality of hardened steel inner race shims are positioned longitudinally on the flats of the inner tube. A plurality of hardened steel outer race shims are positioned longitudinally on the flats of the outer tube. A plurality of needle bearings are disposed between the tubes in between the respective inner and outer race shims. This arrangement allows the two tubes to freely telescope in and out with respect to one another without any significant static friction and also serves to transmit the torsional steering force from the outer tube to the inner tube. This particular system is used extensively today because it can bear a combination of loads comprising very high radial loads and at the same time provide stable and tight rotational motion in steering of the front wheel through the suspension system from the handlebars.
Several problems exist, however, with the practicality of the needle bearing system. First, the preferred embodiment and alternate embodiment described in the prior art for the purpose of adjusting bearing pre-load are impractical, difficult to adjust and expensive. Tolerance stack-up during the manufacturing process makes it difficult and costly to attain the desired bearing pre-load. Assembly time renders the adjustment of multiple shims in the described embodiments impractical and expensive. Excessive bearing pre-load develops objectionable noise, excessive heat and wear, and a non-smooth feeling ride. Insufficient bearing pre-load encourages migration of the bearings and races, resulting in a poorly or non-functioning unit. Consumer preference would be to have the ability to adjust the bearing pre-load depending on the ride and to be able to do it quickly and simply. The prior art designs are impractical as the adjustment mechanisms are located on the outer tube which is fixed inside the head tube of the bicycle and inaccessible without disassembly of the front fork.
Secondly, the highly stressed inner tube must be formed of a material and in a manner such that it bends rather than breaks. High strength steel is commonly used for the inner tube for this purpose, but it is heavy and counter to the consumer""s preference. Lighter materials such as Aluminum in combination with strengthening processes such as shot-peening to strengthen the outer skin have been used as a material for the inner tube. Here-to-for, unfortunately, telescope assemblies whose inner tube connects to the fork crown with greater than 70 mm length of travel have been unable to pass stress testing using the present art as described in the aforementioned patents despite the additional costly process of shot-peening and use of expensive high-strength aluminum alloys. Attempting to increase the diameter of the tubes to add strength is also impractical as larger tube diameters increase cost, obsolete existing factory tooling and is generally counter to the consumer""s aesthetic preference.
A collar on the telescope assembly plays a vital role in the functioning of the telescoping system. The collar has a very fine thread that grips the outer tube. This fine thread makes the mating parts expensive to make, and prone to scrap because of the propensity to cross thread during assembly. Another key function of the lower collar is to prevent the bearing needles from exiting the telescope at the bottom of its excursion. If the bearing needles escape, the entire front fork will come apart. This is prevented by closing down the internal diameter of the collar. However, it cannot be closed down enough to encounter the full length of the bearing cage because the bending of the inner tube when under load may cause it to rub against the collar. A compromise must be made that places severe restrictions on the design, including the outside diameter of the inner tube.
This present invention is an improvement on the telescoping system described in U.S. Pat. No. 5,320,374 and subsequent related patents (U.S. Pat. No. 5,494,302, 5,702,092, 5,924,714, 6,007,056, and 6,155,541). It is a principal object of the present invention to provide an improved mechanism that allows for adjustment of the pre-load applied to the bearing train. In particular, this invention uses a unique method for achieving adjustable race thickness within an extremely tight space without negatively impacting either the outer or inner tube""s strength and with convenient user access for adjustment in the field.
It is yet another object of the invention to provide a low cost collar design which enables consistent and accurate orientation of the collar in relation to the needle bearings preventing the needle bearings from exiting the space in between the inner tube and outer tube. It is yet another object of the invention to provide a collar with access to one of the outer races to allow user adjustment of the compressive load in the field.
It is yet another object of the invention to provide a telescoping assembly with at least 80 mm of travel made of light weight aluminum to qualify or pass life testing while maintaining the present outside geometric configuration of the telescoping system and the connection of the inner tube to the fork crown. This is accomplished through the use of a shaped inner tube and conical relief of the outer tube. The shaped inner tube has additional strength characteristics at critical locations to help reduce breaking under stress. In addition, the shaped inner tube eliminates the need for shot-peening or expensive light weight alloys to increase strength, yet does not increase the size of the entire telescoping assembly. The inner tube remains small in less critical areas so as to minimize weight, and allows sufficient clearance with the collar so as to eliminate interfering with the collar during stress loading. The conical relief allows the flexing of the shaped inner tube under load so as not to interfere or scrape the outer tube.